The present invention relates to a precipitating agent containing thioacetamide for the precipitation of heavy metals from flue gases or waste waters, a method for the production thereof, and the use thereof.
The precipitating agent serves, in particular, for the removing of heavy metals from flue gases and gas scrubbers of power plants employing fossil fuels and of garbage incineration plants. The agent serves furthermore for the precipitation of heavy metals from waste waters of combustion and garbage incineration plants as well as from other waste waters containing heavy metals.
From Federal Republic of Germany 39 07 066 C2 the use of thioacetamide for the decontaminating of sludge sediments is already known. Furthermore, a method of removing heavy-metal ions from aqueous solutions of complex compounds is already known from Federal Republic of Germany OS 26 28 649 in which salts of monothiocarbonic acids are used, possibly in the presence of a phosphate buffer.
From EP 0 043 759 A1, a hydrophilic substrate for the absorption of the metals is already known in which the precipitation of metal sulfides by an aqueous solution of sodium sulfite, potassium sulfite, ammonium sulfite or thioacetamide is also used.
Furthermore, from EP 0 333 672 A1 there is already known a slowly soluble solid product which is formed of substances which are capable or precipitating impurities in the waste waters of dental equipment. It is proposed, inter alia, that an organic compound, for instance thioacetamide, be used which produces hydrogen sulfide upon the hydrolysis.
In power plants which use fossil fuels and in garbage incineration plants which desulfurize and denitrogenize the flue gases in accordance with the wet scrubbing principle, a flue gas is produced which contains in part considerable concentrations of volatile heavy metals, for example arsenic (As), antimony (Sb), mercury (Hg), cadmium (Cd), lead (Pb), tin (Sn), zinc (Zn), and the like.
In particular, the presence of mercury in the flue gas results in great problems for various reasons. Another difficulty is represented by the presence of arsenic upon the firing of lignite, anthracite and heavy oil. Arsenic is contained, in part in considerable quantities, in many types of coal and in a few heavy oils and upon the burning of them necessarily it also enters into the flue gases, namely in gaseous state.
The desulfurization and denitrogenization of flue gases from the above-mentioned plants is effected traditionally in two fundamentally different ways, namely by a so-called dry scrubbing method and by a so-called wet scrubbing method.
The present invention serves, in particular, for use in the wet scrubbing of flue gases of garbage incineration plants and provides a special solution for the problems and goals in that field.
Combustion flue gases from power plants which burn fossil fuels as well as from garbage incineration plants contain, after passage through electrofilters and other fly-dust and particle separators, also gaseous components in addition to fractions of very fine particles. One central problem is represented in this connection by volatile heavy metals, particularly arsenic and mercury. Of the metals mentioned above, all, with the exception of arsenic and mercury, can be easily removed almost completely from the stream of gas merely by contact with water.
This is only imperfectly possible in the case of mercury and arsenic, which, on the one hand, causes measurable residual contents of these metals in the clean gas and, on the other hand, leads in the case of multi-step scrubbing methods, to an entrainment of these substances into the following stages. In multi-stage flue gas scrubbing plants, the first stage is generally operated as "acid scrubbing stage" with feeding of fresh water or else by recycling in order to retain the said heavy metals, hydrochloric acid and other impurities. The second stage operates either directly with milk of lime (emulsion of Ca(OH).sub.2 in water) or, more frequently, with caustic soda solution in order to remove sulfur dioxide from the flue gas. When sodium hydroxide is used, sodium sulfate (Na.sub.2 SO.sub.4) is produced, which thereupon, in a "gypsum stage," is reacted with calcium chloride (CaCl.sub.2) to form gypsum (CaSO.sub.4) and sodium chloride (NaCl).
If arsenic or mercury enters into this SO.sub.2 -scrubbing stage, this necessarily leads to the undesired contaminating of the gypsum, which limits the use thereof or even makes it impossible. At the same time, measurable residual contents of the metals remain in the purified flue gas and thus enter the outer air.
The substantial elimination of arsenic in the flue gas from acid scrubbing stages of two-stage scrubbing systems can generally be effected without difficulty by the addition of suitable arsenic-combining substances. The removal of metals is considerably more problematic when arsenic and mercury or mercury alone are present as substantial impurities in the flue gas since the mercury is present in unstable transition stages of Hg.sup.I Hg.sup.II as a result of the strongly reductive matrix (predominantly SO.sub.2) in the hot acid scrubbing zone of the first scrubber. This Hg passes through the spray scrubbing in predominantly dissolved (more than 90%) but only as part of the total mass, into the aqueous acid phase, the gas being cooled to about 150.degree. C. from its previous value of 180.degree. C. to 230.degree. C. Due to the strongly hydrochloric-acid pH (0.2-1.5), the SO.sub.2 remains substantially dissolved in the flue gas.
In the case of acid scrubbing stages with water recycling, only a small partial stream per hour as removal of salts and sludge from the bottom of the scrubber is replaced by fresh water which leads to an enrichment of the dissolved mercury in the wash water. As a result of this, there is established within the gas-vapor-water phase a mercury equilibrium which is determined by the factors of Hg content of the raw gas, Hg content of the wash solution, temperature of the system, pressure of the system, pH, etc.
In accordance with the laws of physical chemistry, therefore, a substantial or even complete elimination of the mercury from the flue gas cannot be obtained by this method. A precipitation of the Hg as mercuric sulfide (HgS) fails as a result of the strongly reducing action of the hydrogen sulfide (H.sub.2 S) produced upon the use of polysulfide agents in acid solution, it effecting a partial disproportionation of the mercury into Hg.sup.II and Hg.sup.0.
Upon the use of inorganic sulfide and polysulfide substances, there are obtained, depending upon the quantity of the precipitating agent used, between 5 and 30% of the mercury contained therein, as elementary mercury (Hg.sup.0), which is entrained by the flue gas depending on the temperature in accordance with its partial pressure and thus carried along into the SO.sub.2 -scrubbing stage and there either passes into the gypsum or is brought, in part via the clean gas, back into the outer air.
The use of organic sulfur compounds is generally out of the question since, as a rule, they are only slightly, if at all, cohesive in acid solution and they form organic sulfur-mercury adducts which are split again thermally in the hot flue gas zone of the mercury. The limit values for mercury in the pure gas are at present 50 .mu.g/Nm.sup.3 Hg, which, however, can be maintained only with difficulty without precipitating agent. Furthermore, they are further reduced before long by the changing of the TA air so that a solution of the problem described becomes evermore urgently necessary.
The object therefore arises of providing a precipitating agent which under the action of heat, within the temperature range of 50.degree. C. to 250.degree. C., precipitates arsenic and mercury from flue gases and/or acid scrubber waste streams and with which the formation of hydrogen sulfide or other strongly reducing substances is suppressed. Another object of the present invention is to provide a method for the production of the precipitating agent and its use in a gas scrubbing solution system in which arsenic and mercury are precipitated either directly from the hot stream of gas or from the wash-solution stream of a temperature of 40.degree. C. to 90.degree. C. without resulting in the formation of elementary Hg. The precipitating agents and the methods of production and use are to be such that they can be used, in particular, for flue gas cleaning processes with two-stage scrubbing systems including a gypsum salt stage without any negative effects on the air, water or earth resulting upon their production or use due, for instance, to the liberation of dangerous reaction products.